The present invention relates to a method of recovering valuable metal from scrap containing conductive oxide as represented by an ITO scrap of a spent indium-tin oxide (ITO) sputtering target or an ITO scrap such as ITO mill ends (hereinafter collectively referred to as “scrap containing conductive oxide” in this specification). Incidentally, the phrase “recovery of valuable metal” as used herein shall include the recovery of metals having valuable metal as its component, as well as solutions, alloys, hydroxides, oxides, suboxides and peroxides containing such metals.
In recent years, an indium-tin oxide (In2O3—SnO2: generally known as ITO) sputtering target is being widely used in numerous electronic components; for instance, for a transparent conductive thin film of an LCD device or a gas sensor. In many cases, however, it is used as a result of a thin film being formed on a substrate or the like by employing the sputtering method as the thin film forming means. ITO is a representative oxide with conductive property.
Oxide with conductive property (conductive oxide) is not limited to ITO, and there are numerous conductive oxides including IGZO (In—Ga—Zn—O composite oxide), ZnO, Zn—SnO2, and SnO2, and the like. These composite oxides are similarly used for preparing a target, sputtering the target to form a thin film, and thereby used in various electronic components.
It is true that the sputtering method as the thin film forming means is superior, but when a sputtering target is used to form a transparent conductive thin film or the like, the target will not be consumed uniformly. A portion of the target with severe consumption is generally referred to as an eroded portion, and the sputtering operation is continued until immediately before a backing plate supporting the target becomes exposed due to the further consumption of the eroded portion. This target is subsequently replaced with a new target.
Accordingly, a spent sputtering target will have numerous non-eroded portions, that is, unused portions of the target, and all of these conductive oxides become scrap. Moreover, even during the production of a sputtering target, scraps (mill ends) will arise from abrasive powder, cutting powder and the like.
Since expensive materials are used as the conductive oxide sputtering target material, the process of recovering raw materials from scrap materials is being generally performed. As this kind of metal recovery method, conventionally, a method that combines wet refining such as the acid solution process, ion exchange method, and solvent extraction method has been used.
For instance, in the case of an ITO scrap, there is a method of subjecting such ITO scrap to cleansing and pulverization, thereafter dissolving this in hydrochloric acid, precipitating and eliminating impurities such as zinc, tin, lead and copper as sulfide by passing hydrogen sulfide through the solution, thereafter adding ammonia to neutralize the solution, and recovering the resulting indium hydroxide.
For instance, in the case of an ITO scrap, there is a method of subjecting such ITO scrap to cleansing and pulverization, thereafter dissolving this in hydrochloric acid, precipitating and eliminating impurities such as zinc, tin, lead and copper as sulfide by passing hydrogen sulfide through the solution, thereafter adding ammonia to neutralize the solution, and recovering the resulting indium hydroxide.
Nevertheless, the indium hydroxide obtained with the foregoing method has inferior filtration property, requires long operational process, and contains large amounts of impurities such as Si and Al. In addition, with the created indium hydroxide, since the grain size and grain size distribution will vary depending on the neutralization condition, maturization condition and other conditions, there is a problem in that the characteristics of the ITO target cannot be stably maintained upon subsequently manufacturing such ITO target. The same applies to other conductive oxides.
Conventional technology and its advantages and disadvantages are described below taking ITO as the representative example.
As one example of such conventional technology, there is an etching method of a transparent conductive film including the steps of reducing an ITO film deposited on a substrate based on an electrochemical reaction in the electrolyte, and dissolving the reduced transparent conductive film in the electrolyte (refer to Patent Document 1). However, the object of this method is to obtain a mask pattern with high precision, and relates to technology that is different from the recovery method.
There is also technology of separating, in the electrolyte, the impurities contained in an In brazing filler material used in the bonding with the backing plate as pretreatment for recovering valuable metal from ITO (refer to Patent Document 2). Nevertheless, this method does not relate to technology of directly recovering valuable metal from ITO.
Moreover, upon recovering indium from an intermediate obtained as a by-product of the zinc refining process or an ITO scrap, disclosed is technology of separating tin as halogenated stannate, performing reduction treatment with hydrochloric acid or a nitric acid solution, subsequently adjusting the pH of this aqueous solution to 2 to 5, reducing metallic ions of iron, zinc, copper, thallium and the like in order to obtain a substance that will not precipitate easily, and separating the indium component in the aqueous solution (refer to Patent Document 3). With this technology, however, there is a problem in that the refining process is complicated, and a superior refining effect cannot be expected.
Further, as a method of recovering high purity indium, disclosed is technology of dissolving ITO in hydrochloric acid, adding alkali thereto to make the pH 0.5 to 4, eliminating tin as hydroxide, subsequently blowing hydrogen sulfide gas in order to eliminate hazardous materials such as copper and lead as sulfide, and electrowinning indium metal by performing electrolysis using the obtained solution (refer to Patent Document 4). There is a problem with this technology in that the refining process is complicated.
In addition, proposed is a method of dissolving an ITO indium-containing scrap in hydrochloric acid to obtain an indium chloride solution, adding a sodium hydroxide solution to this solution to eliminate tin as tin hydroxide, additionally adding a sodium hydroxide solution after the elimination to obtain indium hydroxide, filtering the obtained indium hydroxide, obtaining indium sulfate from the filtered indium hydroxide, and obtaining indium by electrowinning the indium sulfate (refer to Patent Document 5). Although this is an effective method with a significant refining effect, there is a drawback in that the process is complicated.
Also proposed is a method of recovering indium including the steps of dissolving an ITO indium-containing scrap in hydrochloric acid to obtain an indium chloride solution, adding a sodium hydroxide solution to the indium chloride solution to eliminate tin contained in the scrap as tin hydroxide, substituting indium with zinc from the solution after eliminating the tin hydroxide, and thereafter recovering indium (refer to Patent Document 6). Although this is also an effective method with a significant refining effect, there is a drawback in that the process is complicated.
Additionally disclosed is a method of recovering metallic indium including the steps of extracting suboxide-containing cast scrap floating on molten metal indium and introducing this into an atmosphere furnace, vacuating the furnace once, thereafter introducing argon gas, heating the furnace to a prescribed temperature and reducing the suboxide-containing cast scrap (refer to Patent Document 7).
Although this is in itself an effective method, there is a drawback in that this is not a fundamental recovery method of conductive oxide scrap.
In light of the above, a method that is efficient and has a versatile recovery process is being sought.    [Patent Document 1] Japanese Patent Laid-Open Publication No. S62-290900    [Patent Document 2] Japanese Patent Laid-Open Publication No. H8-41560    [Patent Document 3] Japanese Patent Laid-Open Publication No. H3-82720    [Patent Document 4] Japanese Patent Laid-Open Publication No. 2000-169991    [Patent Document 5] Japanese Patent Laid-Open Publication No. 2002-69684    [Patent Document 6] Japanese Patent Laid-Open Publication No. 2002-69544    [Patent Document 7] Japanese Patent Laid-Open Publication No. 2002-241865